1. Field of the Invention
The present invention relates to a safety switch assembly and in particular to a safety switch assembly capable of indicating whether or not a safety enclosure protecting an item of machinery has been correctly closed.
2. Description of the Related Art
Safety switch assemblies are known which comprise first and second elements intended to be mounted on relatively movable components of the safety enclosure, the first and second elements being in a predetermined relative position adjacent to each other when the enclosure is in a closed condition and spaced apart when the enclosure is opened. The known safety switch assemblies provide an output indicating whether or not the enclosure has been correctly closed, the output being used to disable machinery protected by the enclosure when the enclosure has not been properly closed.
In one known safety switch assembly, a permanent magnet is mounted on one element and a magnetically operated switch is provided on the other element. When the two elements are close together, the permanent magnet causes a magnetic reed switch to close, thereby enabling operation of the machine. When the two elements are spaced apart the output disables the machine. The known magnetically operated safety switch assemblies provide acceptable characteristics but it is relatively easy for a machine operator to disable a safety switch assembly simply by placing a permanent magnet adjacent the magnetically operated switch so that the safety switch assembly is in effect rendered inoperative.
In another known safety switch assembly, one of the elements is provided with a tuned circuit which is energised to generate an electromagnetic field in its immediate vicinity. The other element supports a passive tuned circuit which when close to the active element is electromagnetically coupled to it so that an output derived from the active element indicates the presence of the passive element. The problem with this arrangement is that the circuitry is vulnerable to interference from electromagnetic signal sources in its vicinity and can provide an erroneous output. In particular, where the active tuned circuit is driven by a signal generated remotely, conductors connecting the signal source to the active element can themselves pick up spurious signals with the result that the output of the safety switch assembly cannot be relied upon.
It is an object of the present invention to obviate or mitigate the problems associated with known safety switch assemblies as outlined above.